Not the biggest for long
You have to do your willy-waving while you can in this game - the ELT is already under construction and will be a 39.3m monster when it achieves first light in 2024.
The planned largest optical telescope in the world, the Giant Magellan Telescope looks to go ahead, with its consortium signing off on the US$500 million build cost. The telescope is ambitious engineering: seven mirrors spanning a total of 25 metres, which the project says will focus “six times” the amount of light collected …
That is Brian Schmidt for you not Bernhard Woldemar Schmidt the one-armed opticial genius.
Nowadays its more of a get as high as possible so there ain't so much air to get through! Its nice and high up where they're building these monsters so you get less weather, less air to reflect back any street lights, less wobbly air to correct for. There are places in europe that high (2500m+) but the vibration from snowboarders hitting the doors would make it impractical.
I'd be interested to see if their are any figures for less brain power in the scientists who work at this height - I notice problems standing up these days.
It's not just the altitude - locating the devices in Chile is to benefit from the extraordinarily dry Atacama climate. Not only are there few clouds to interrupt viewing, but the absence of water molecules in what little atmosphere is above them allows better performance especially in the infra-red.
PS: There ain't no scientists out there - they're all crouched over their internet-connected terminals back here in Europe when they're not arguing their latest theory at some fancy conference. It's only the poor bloody engineers assembling them and positioning them out there (and, yes, they do need supplementary oxygen).
PPS: These monsters may seem somewhat poor-value-for-money when we have so many homeless, but in comparison with what governments like to spend on things that go bang.........
Apart from the altitude, dry air, and low light pollution, Chile has two big advantages for astronomy - it's politically stable, and even more importantly it's in the southern hemisphere, unlike all those North American and European observatories, so sees a different region of the sky.